Decoherence describes the tendency of quantum sub-systems to dynamically lose their quantum character . This happens when the quantum sub-system of interest interacts and becomes entangled with an environment that is traced out . For ordinary macroscopic systems , electromagnetic and other interactions cause rapid decoherence . However , dark matter ( DM ) may have the unique possibility of exhibiting naturally prolonged macroscopic quantum properties due to its weak coupling to its environment , particularly if it only interacts gravitationally . In this work , we compute the rate of decoherence for light DM in the galaxy , where a local density has its mass , size , and location in a quantum superposition . The decoherence is via the gravitational interaction of the DM overdensity with its environment , provided by ordinary matter . We focus on relatively robust configurations : DM perturbations that involve an overdensity followed by an underdensity , with no monopole , such that it is only observable at relatively close distances . We use non-relativistic scattering theory with a Newtonian potential generated by the overdensity to determine how a probe particle scatters off of it and thereby becomes entangled . As an application , we consider light scalar DM , including axions . In the galactic halo , we use diffuse hydrogen as the environment , while near the earth , we use air as the environment . For an overdensity whose size is the typical DM de Broglie wavelength , we find that the decoherence rate in the halo is higher than the present Hubble rate for DM masses m _ { a } \lesssim 5 \times 10 ^ { -7 } eV and in earth based experiments it is higher than the classical field coherence rate for m _ { a } \lesssim 10 ^ { -6 } eV . When spreading of the states occurs , the rates can become much faster , as we quantify . Also , we establish that DM BECs decohere very rapidly and so are very well described by classical field theory .